x402
Pattern for selling digital goods over HTTP using 402 Payment Required semantics, machine-readable price fields, and on-chain settlement (on LVL: Base USDC).
HTTP 402
Status code Payment Required. On LVL pay challenges, a 402 with amount and payTo is expected business logic, not a random server failure.
Skill
A versioned capability pack listed in the catalog with free evaluation and optional sealed unlock.
Outline
Free JSON (or docs) describing the skill before payment. Primary evaluation surface for agents and humans.
Sealed pack
Paid implementation files returned after verified payment. Distinct from public teasers.
Canary
Tiny-priced skill (on LVL, often $0.05 agent-x402-first-buy) used to prove payment and unlock rails.
Tripwire
Low but meaningful price (for example around $0.99) that filters serious buyers after the canary.
payTo
Address that must receive the USDC transfer for a challenge to verify.
maxAmountRequired
Atomic token amount required by the challenge (USDC 6 decimals on Base).
X-PAYMENT
Client-presented payment proof on unlock POST, typically including txHash and skill id.
Unlock
Server action that verifies settlement and returns sealed pack files.
Idempotent re-redeem
Using the same valid payment proof to retrieve files again without a second transfer.
Escrow (LVLEscrow)
Contract-assisted hold/release path used in open marketplace flows so funds are not blindly trusted to a raw peer transfer alone.
Proof ledger
Public confirmed unlock records at /api/proof. Source of truth for volume claims.
Base
EVM network with chain ID 8453 used for LVL USDC settlement.
Phrase book for support tickets
When you write support or agent logs, prefer: "Base USDC transfer 0xabc confirmed; unlock POST returned 200; sealed files written." Avoid: "payment kinda worked." Precise language maps to proof and BaseScan faster.
When a seller writes listing copy, prefer: "Unlock returns SKILL.md and runbook.json; needs Python 3.11; no hosted API." Avoid: "revolutionary AI sauce." Specificity is a trust signal.
Additional terms
Atomic units: Integer token amounts scaled by decimals (USDC 6).
Allowlist: Explicit set of addresses or skill ids an agent may interact with.
Challenge: Server response that states payment requirements for a resource.
Confirmations: Blocks mined after your tx before you trust it as final under your policy.
Curated catalog: First-party or tightly controlled inventory in catalog.json.
Deep link: Marketplace URL with buy and pay query parameters for a skill id.
Flagship: High-priced multi-capability pack such as revenue OS style products.
Gas: Network fee paid in Base ETH to execute transactions.
Listing: HTML product page under /listings/{id}/ with schema.org markup.
Machine entry: /api/shop and related JSON doors for agents.
Open marketplace: Seller-upload path with moderation and possible escrow.
Outline: Free evaluation document or JSON for a skill.
Premium shelf: Higher priced or featured subset of skills.
Re-redeem: Unlock again with existing proof.
RPC: Endpoint your wallet or agent uses to talk to Base.
Schema.org Product: Structured data for search engines and careful parsers.
Skill id: Stable string primary key for commerce.
Tripwire: Low price point that filters casual buyers after canaries.
Treasury: Platform payTo address for first-party unlocks.
Volume USDC: Sum of confirmed unlock amounts; not click counts.
How to use this glossary
When an agent log says "challenge accepted but unlock failed," map each word here, then jump to the x402 and agents HTTP guides. Shared vocabulary prevents expensive miscommunication between humans and bots.
Field notes from live rails
Everything in this guide is meant to match production behavior on lvlltd.com: free outlines at /skills/{id}/outline.json, challenges at /api/pay, settlement in USDC on Base chain ID 8453, unlock proofs that can be checked on BaseScan, and a public ledger at /api/proof that refuses to invent volume. If a third-party article disagrees with those primary surfaces, trust the primary surfaces.
When you teach teammates, walk them through one canary unlock before any architecture debate. Shared scars on a $0.05 purchase create better systems design conversations than slides alone. Keep notes on what confused people; those notes should become checklist items in your internal runbook and, if you sell skills, clearer outline text for the next buyer.
Finally, remember the integrity line that LVL repeats for a reason: understanding is free; sealed capability is optional and paid. Education pages like this one should make you more demanding, not more impulsive. Read outlines. Allowlist addresses. Prefer small spends first. Verify. Then scale.
Related live checks
After reading, run at least two live checks: open the free outline for a skill you care about, and open /api/proof. Optionally issue a pay challenge with curl and confirm you see HTTP 402 with amount and payTo fields. Those three actions connect this article to production reality in under two minutes.
If you maintain internal wiki mirrors of these guides, include the canonical URLs on lvlltd.com so agents and humans can detect drift. Canonicals and llms.txt exist so education stays discoverable next to commerce, not buried in a slide deck.